IT’s all set up around the TTY port and that’s really what you want to know is functioning. Checking that can be a little bit involved. In the very eary days and when we moved from Pi2 to Pi3 I seem to remember we eeded to change the value of the basic MIDI clock value that set up the TTY port, but that really is ancient history.
A ‘simple’ GPIO sketch using a Raspian SSD would at least allow you to see if you can get the Opto-Isolator actually controlling the GPIO pin that MIDI comes in over.
As @riban says you probably want to check the opto-isolator actually does the sorts of things it’s meant to.